1) Field of the Invention
The invention relates to a process for preparing powder compositions which comprise organosilicon compounds and are redispersible in water.
2) Background Art
Water-redispersible powder compositions comprising organosilicon compounds are known, for example, from EP-A 228657 (U.S. Pat. No. 4,704,416) and EP-A 279373.
EP-A 228657 describes redispersible powders comprising organosilicon compounds, which powders are prepared by spray drying in the presence of a water-soluble organic polymer. For this purpose, a mixture of the organosilicon compounds with the organic polymer is atomized. To improve the blocking resistance of the redispersible powder, it is proposed that, during spray drying, the total amount of antiblocking agent be metered into the spray dryer separately from the aqueous mixture to be dried.
EP-A 279373 relates to water-redispersible organo(poly)siloxane-containing powders which are prepared by spray drying in the presence of a water-soluble organic polymer having a cloud point of from 35.degree. C. to 98.degree. C. To improve the blocking resistance, antiblocking agents are added to the powder. For this purpose, the following three alternatives for adding the antiblocking agent are described: the antiblocking agent is added together with organosilicon polymer and organic polymer to the aqueous mixture to be dried and is atomized as a mixture; it is metered in separately from the aqueous mixture during spray drying; it is metered into the lower part of the spray dryer before exit of the dried powder.
A further possible way of adding antiblocking agents to dispersible powders is to add them to the dried dispersible powder in a separate apparatus.
The methods mentioned have the following disadvantages: when the antiblocking agent is added during spray drying, in the aqueous mixture to be dried, the antiblocking agent cannot fully develop its activity since it is, like the organosilicon compound, present in microencapsulated form or is embedded in the water-soluble organic polymer. If the total amount of antiblocking agent is added separately therefrom at the top, a considerable part of the antiblocking agent is entrained in the air stream which flows out and has to be held back by means of expensive air filter units. When the total amount of the antiblocking agent is mixed in before exit of the dried powder, together with the cold air fed in to convey the dried powder, considerable encrustations and deposits caused by blocking of the powder occur on the dryer wall, on the cone and on the exit pipe from the spray dryer, which leads to long downtimes for cleaning. Mixing in the antiblocking agent after drying in a separate apparatus likewise leads to encrustations and deposits in the mixing apparatus.
It is therefore an object of the invention to provide a process for preparing redispersible powders comprising organosilicon compounds, which gives free-flowing and blocking-resistant powders using very small amounts of antiblocking agents, without the above-mentioned disadvantages occurring.